Here's a reply I posted to a similar question last week:
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I believe the reason for doing a scant 1/4" is that if you were to sew EXACTLY on the 1/4" mark, your finished seam allowance will be slightly larger than 1/4" - which means the other part of the fabric, the part that's in the block, will be a little smaller than it should be.
This is due to the fact that the thread you are sewing with takes up a little bit of room, and when you press the seam to one side you lose another little bit.
If you are doing blocks with large pieces and just one or two seams it won't really matter. But if you have lots of little pieces and many seams, they all add up. For example, if you are off 1/32" on each seam, and you have 8 seams in a block, your block will end up 1/4" too small.
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And here's a link to a tute on testing your 1/4 seam for accuracy:
http://www.quiltingboard.com/t-89997-1.htm